Apparatus for frame-to-frame comb filtering composite TV signal

ABSTRACT

A frame-to-frame comb filter for separating chrominance and luminance signals from composite video signals produces objectionable double images around the edges of moving objects. The double images are substantially removed by low pass filtering the combed chrominance signal and combining the low pass filtered chrominamce signal with the combed luminance signal.

BACKGROUND OF THE INVENTION

This invention relates to frame-to-frame comb filters for separating theluminance (Y) and chrominance (C) components of composite televisionsignals and more particularly to means for correcting signal distortiondue to the occurrence of motion between successive video frames in sucha comb filter system.

Electronic signals which are periodic in nature may be processedadvantageously by storing replicas of the signal which are separated intime by the repetition period and then combining the stored replicas toenhance information content of the signal. For example, conventionaltelevision broadcast systems (and most video record/playback systems)are arranged so that much of the brightness (luminance) informationcontained in an image is represented by signal frequencies which areconcentrated about integral multiples of the horizontal line scanningfrequency. Color (chrominance) information is encoded or inserted in aportion of the luminance signal spectrum around frequencies which liehalfway between the multiples of line scanning frequency (i.e., at oddmultiples of one-half the line scanning frequency).

Chrominance and luminance information can be separated and detailinformation may be enhanced by appropriately combing the compositesignal spectrum. Known combing arrangements take advantage of the factthat the odd multiple relationship between chrominance signal componentsand half the line scanning frequency causes the chrominance signalcomponents for corresponding image areas on successive lines to be 180°out of phase with each other (so-called interlaced frequencycomponents). Luminance signal components for corresponding image areason successive lines are substantially in phase with each other (andcomprise non-interlaced components).

In a comb filter system, one or more replicas of the compositeimage-representative signal are produced which are time delayed fromeach other by at least one line scanning interval (a so-called 1-Hdelay). The signals from one line are added to signals from a precedingline, resulting in the cancellation of the interlaced frequencycomponents (e.g., chrominance) while reinforcing the non-interlacedfrequency components (e.g., luminance). By subtracting the signals fortwo successive lines (e.g., by inverting the signals for one line andthen adding the two), the non-interlaced frequency components arecancelled while the interlaced frequency components are reinforced.Thus, the luminance and chrominance signals may be mutually combed andthereby may be separated advantageously.

The foregoing combing process is possible because of the inherent highlevel of signal redundancy line-to-line in a typical video scene. If oneconsiders a stationary picture, and a particular pixel of that scene, itwill be appreciated that the level of redundancy of the signalrepresenting that pixel on a frame-to-frame basis will be substantiallyone hundred percent. Thus, if combing were performed on a frame-to-framebasis, the percentage of residual luminance (chrominance) signal in thecombed chrominance (luminance) signal will be significantly reduced overa line-to-line combed signal. More particularly, there will be, forexample, no vertical detail accompanying the combed chrominance signal.By design, the chrominance signal of an NTSC composite signal issynchronized to have a 180 degree phase relationship from frame to framepermitting comb filtering on a frame basis. The luminance component maytherefore be extracted by linear addition of the composite signal fromsuccessive frames and the chrominance component may be extracted bylinear subtraction of the composite signal from successive frames. Theresponse characteristic of a line-to-line comb filter has nulls or teethspaced at line frequency or 15.734 KHz intervals while theframe-to-frame filter response has teeth at 30 Hz intervals. The closerteeth or null spacing of the frame comb gives rise to a more completecombing process in all spatial directions than the interline comb.

In the frame-to-frame combing process, non-stationary objects give riseto distortions in the reproduced images. These distortions are due toincomplete cancellation of the luminance and chrominance components inthe combed chrominance and combed luminance signals respectively (crosscomponents) as well as an effective bandwidth reduction of the luminancesignal in the regions of motion. Differences in scene content due toobject motion or panning, that occur in the time frame of one-thirtiethof a second or faster are characterized as being visual motion of thescene content. The motion induced distortions created by theframe-to-frame combing process are two dimensional and are due entirelyto interframe scene motion. The distortions are observable in both thehorizontal and vertical directions in the plane of the image and aremanifested as dual images in the reproduced scenes. The dual images areseparated by an amount corresponding to the rate of motion and may beaccompanied by incorrect hue at the edges of the moving objects.

In contrast, interline comb filters create one dimensional distortionswhich are due to vertical structure within a scene even if stationary.Distortions attendant the interline combing process are manifested as aneffective lowering of the bandwidth of the signal representing verticalscene structure and a softening of vertical edges of the scene content.

It is an object of the present invention to reduce the objectionableeffects produced by motion in a frame-to-frame combing system.

BRIEF SUMMARY OF THE INVENTION

The present invention is a frame-to-frame comb filter including a delayelement for storing successive frames of composite video signal. Alinear summing circuit having first and second input terminals coupledto receive current video signal and video signal delayed by one frametime produces a combed luminance component of the composite videosignal. A linear subtracting circuit having first and second inputterminals coupled to receive current video signal and video signaldelayed by one frame time produces a combed chrominance component of thecomposite video signal. A linear phase low pass filter is seriallyconnected with the linear subtracting circuit to substantially rejectsignals occupying the chrominance signal frequency spectrum and to passresidual luminance signal included in the combed chrominance signal. Asecond summing circuit having first and second input terminals coupledto receive the combed luminance component and the signal processed bythe low pass filter produces a luminance signal substantially free ofartifacts resulting from scene motion between frames of the videosignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a frame-to-frame comb filter known in theart of TV signal processing.

FIGS. 2a-e are amplitude versus time waveforms associated with the FIG.1 circuit.

FIG. 3a, 3b, and 3c are block diagrams of a frame-to-frame comb filterembodying the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a frame-to-frame comb filter. Forpurposes of illustration, the apparatus will be described in terms ofseparating luminance and chrominance signals from a composite TV signal,however, the apparatus is not limited to this particular application.For example, the apparatus of FIG. 1 may be used to comb filterluminance signal for insertion of chrominance signal therein in theformation of a composite video signal.

In FIG. 1, a storage element 10 having the capacity to store theelectrical manifestation of one frame (e.g., two fields of NTSC videoformat) of video signal, sequentially accepts signal from connection 11and outputs the same signal onto connection 14 delayed by the durationof one frame period. The element 10 may comprise a digital memory, e.g.,a random access memory (RAM) with the appropriate support electronics oran analog memory, e.g., a serial charge transfer device (CTD) such as aCCD with its necessary supporting circuitry. Conceptually it does notmatter whether element 10 is a digital or analog circuit, however, itwill be assumed that it is digital in nature. The remaining circuitelements incorporated therewith will therefore be considered to bedigital in nature to obviate digital-to-analog signal conversion betweenthe circuit elements.

Input signal from connection 11 and delayed signal from connection 14are summed together in the ADDER circuit 12. The luminance components ofsignal being in phase add while the chrominance components being 180degrees out of phase cancel to produce a substantially chrominance freeluminance signal Y at connection 15. Input signal and delayed signal arealso applied to the SUBTRACTOR circuit 13 wherein the chrominancecomponents of the two frames sum constructively while the luminancecomponents cancel to produce a substantially luminance free chrominancesignal on connection 16 (at least when successive frame signals arerecords of the same stationary scene).

Referring to FIG. 2, the effects of motion on the aforedescribedframe-to-frame combing process is graphically illustrated. FIG. 2arepresents a portion of the composite signal at an arbitrary timeinterval. FIG. 2b represents the composite video signal from the samescene exactly one frame period later in time and wherein some motion hasoccurred in the scene. In waveform 2a, assume that the signal componentbetween times T1 and T3 represents an object having luminance andchrominance signal parameters L2 and C2 respectively and is located inan area of uniform scene content having luminance and chrominance signalparameters L1 and C1 respectively. Between frames, the object movesspatially causing the video signal representation of the object totranslate temporally, i.e., to fall between times T2 and T4 in FIG. 2b.The relative signal parameters L and C of the luminance and chrominancecomponents of signal remains the same between the FIG. 2a and 2bwaveforms though there exists a time shift with respect to the objectrepresented by L2, C2. The chrominance component of the signal, however,has a 180 degree phase relationship between waveforms 2a and 2b (i.e.,between frames). The sum and difference of the FIG. 2a and 2b waveformsare represented by the waveforms 2c and 2d respectively. It is seen thatfor those segments of time in which the absolute value of thechrominance vector component between the 2a and 2b waveforms is thesame, substantially complete cancellation of the chrominance componentresults from summing the two waveforms (FIG. 2c). Similarly, where theamplitude of the luminance component of the signal is the same betweenthe two waveforms (2a and 2b) substantially complete cancellation of theluminance component results from a subtraction of the two waveforms FIG.2d. FIGS. 2a, 2b, 2c and 2d are representative of signals occurringrespectively at connections 11, 14, 15 and 16 of the FIG. 1 circuit.

Over the period where motion occurs between frames, i.e., between timesT1 and T2 and times T3 and T4, unequal signals are being differentiallysummed, and incomplete cancellation of the undesired component occurs.

In addition, motion distorts the desired signal. The motion causes anaveraging of the signal over the periods where motion occurs. Theseaverages are represented in FIG. 2c by the signal segments denoted(L1+L2) which is equivalent to (2L1+2L2)/2. Normally the signal from thetwo frames will be weighted by a factor of 1/2 before being combined.The amplitude of the combed signal will be 1/2 the values shown and thesignal during the motion period will be (L1+L2)/2. The averages (L1+L2)in the luminance signal tend to replicate the signal transitions toproduce a double or phantom image which circumscribes the moving object.The averages (C1+C2) in the combed chrominance signal during periods ofinterframe motion tends to distort the hue around the moving object.

Low pass filtering the combed chrominance signal of FIG. 2d produces thesignal shown in FIG. 2e. In FIG. 2e, the chrominance signal spectrum hasbeen removed, leaving only the uncancelled luminance component resultingfrom the interframe motion. If the signal of FIG. 2e is added orsubtracted to the signal of FIG. 2c, the edges of the luminance signalwill be restored, producing a luminance component equivalent to onewhich has not been combed. Addition of the FIG. 2e signal will restorethe luminance signal of FIG. 2c between the times T1 to T2 to the levelof 2L2 and reduce the luminance signal between times T3 and T4 to thevalue of 2L1 as indicated by the broken lines.

The circuitry shown in FIG. 3a is a frame-to-frame comb filter withfacility ior correcting artifacts such as double images or smearing dueto interframe motion. In FIG. 3, elements designated with the samenumerals as elements in FIG. 1 perform identical functions. Thus, at theconnections 15 and 16 of the FIG. 3a circuit, combed luminance andchrominance signals are respectively produced, which signals containcross components as well as imperfectly combed signal components due tointerframe motion.

The combed chrominance signal at connection 16 is applied to a linearphase low pass filter 27 to remove the chrominance signal spectrumleaving the uncancelled low frequency luminance components. The low passfiltered signal from filter 27 is added back to the combed luminancesignal from connection 15 in ADDER circuit 31 to produce correctedluminance Y_(c) at connection 32. The residual uncancelled chrominancecomponent in the combed luminance signal from connection 15 tends onlyto create a second order distortion which is not observable except onclose inspection of the displayed image.

The low pass filter 27 must be linear phase in order to properlyreinsert the motion detail into the combed luminance signal. If themotion detail is not properly reinserted, due to phase differentialsbetween the motion affected segments of the combed luminance signal andthe motion detail extracted from the combed chrominance signal,transient response will be adversely affected.

While the residual chrominance (C2-C1, C1-C2) in the luminance channeldue to interframe motion does not produce significant artifacts in thereproduced image, the averaged or transitional chrominance signal(C1+C2) in the combed chrominance signal component, occurring duringmotion periods, may produce particularly objectionable colordistortions. Remembering that the chrominance signal is a vectorquantity, the vector sum C2+C1 during motion periods may produce colorscompletely different from those represented by either of the signals C1or C2. Thus, to retain color purity at the edges of moving objects, thechrominance component must be corrected for motion induced errorsgenerated in the comb filtering process.

The information necessary for correcting the combed chrominance signalresides in the combed luminance signal and may be reinserted in a mannersimilar to the method for restoring the motion detail to the combedluminance signal. However, the chrominance restoration should not be acontinuous process, but should only be performed during motionsequences. The reason for this is the relatively large energy density ofthe luminance signal compared to the cross component of the chrominancesignal. Continuous addition of the combed luminance signal to the combedchrominance signal would effectively defeat the combing process.However, the addition of the combed luminance signal to the combedchrominance signal during periods of motion produces a preferable signalto no chrominance reinsertion. Since it is normally necessary tobandpass filter the combed chrominance signal to eliminate the residualluminance signal, i.e., luminance motion detail, the combed luminancecomponent added to the combed chrominance is substantially eliminatedfrom the corrected chrominance signal. Only the luminance componentsresiding in the spectral band of the chrominance signal remain in thecorrected chrominance signal and the energy of the luminance signal inthis band is relatively low.

The luminance signal is comprised of constituents of the chrominancesignal, i.e., red, blue and green signals, and therefore the amplitudeof the luminance signal is indicative of the amplitude of thechrominance signal. The occurrence of signal in excess of apredetermined value emanating from low pass filter 27 indicates theoccurrence of motion and the amplitude of this signal is indicative ofthe amplitude of the chrominance errors. Chrominance signal errors largeenough to cause objectionable artifacts in the reproduced image cantherefore be detected by measuring the amplitude of the luminance signalpassed by the low pass filter 27.

In FIG. 3a a motion detector 28 coupled to the output connection of lowpass filter 27 compares the low pass filtered combed chrominance signalagainst a threshold value. The threshold value is set at a levelcorresponding to a signal level which has been established to causeobjectionable chrominance artifacts. When the low pass filtered signalfrom filter 27 exceeds the threshold value, the motion detector 28generates a control signal which closes the switch 29.

FIGS. 3b and 3c are illustrative examples of analog signal and digitalsignal motion detectors respectively. The analog version 28' comprises ahigh gain differential amplifier 40 and a source of reference potential41. Signal from filter 27 is applied to a non-inverting input terminal42 of amplifier 40 and signal from reference source 41 is applied to theinverting terminal of amplifier 40. When the potential at terminal 42exceeds the reference, the output signal from amplifier 40 goes highotherwise it remains low. Coupled to the output terminal of amplifier 40is an N-type field-effect transistor 29' which may be implemented inplace of switch 29 of FIG. 3a. A high potential applied to the gate (G)of the transistor closes the switch and a low potential opens theswitch.

The digital version of FIG. 3c comprises a binary reference source 45, asubtraction circuit 43 (comparator), a sign detector 44 and an ANDcircuit switch 29". (In the drawing double line interconnections connoteparallel bit lines.) Subtractor 43 subtracts the reference number (45)from binary signal at the output 42 of filter 27. The difference valueis examined by a sign circuit 44 to determine if it is positive ornegative. If the difference is positive, the SGN circuit produces apositive output pulse which enables AND circuit 29" to pass the signalon its data lines. Note, if the subtractor performs twos complementarithmetic, the sign circuit may be eliminated and replaced with aninverter circuit directly connected between the most significant bit,i.e., the sign bit, of the subtractor output connections and the ANDcircuit.

Switch 29 selectively couples the combed luminance signal fromconnection 15 to one input of ADDER circuit 30. A second input of ADDERcircuit 30 is directly coupled to receive the combed chrominance signalfrom connection 16. When motion detail exceeding the threshold value isdetected, switch 29 is closed and the combed chrominance and luminancecomponents are added in the ADDER circuit 30. When no motion detail isdetected, switch 29 remains open and the combed chrominance signal fromconnection 16 is passed unaltered by the ADDER circuit.

The output signal from ADDER circuit 30 is coupled to a linear phasebandpass filter 34 which passes only the chrominance signal frequencyspectrum.

In the following claims, the term "image period" is defined to mean thatportion or segment of a video signal which includes sufficientinformation to produce a scene or picture which fills a kinescopedisplay tube.

What is claimed is:
 1. A comb filter for processing TV signalscomprising:a delay means for delaying said TV signal by one imageperiod; first means for combining said TV signal and delayed TV signalfrom said delay means to produce the sum thereof correspondingsubstantially to a first component from said TV signal; second means forcombining said TV signal and delayed TV signal from said delay means toproduce the difference thereof corresponding substantially to a secondcomponent from said TV signal; filter means having an input terminalcoupled to said second means, and having an output terminal, said filtermeans having a transfer characteristic for substantially rejecting saidsecond component of said TV signal; and third means for combining thefirst component of signal with filtered signal from the output terminalof said filter means to correct the first component of signal for errorstherein due to interimage motion.
 2. A comb filter for processing acomposite TV signal including chrominance and luminance components,comprising:a delay means for delaying said TV signal by at least oneframe period; first means for combining said TV signal and delayed TVsignal from said delay means to produce the sum thereof correspondingsubstantially to said luminance component from said TV signal; secondmeans for combining said TV signal and delayed TV signal from said delaymeans to produce the difference thereof corresponding substantially tosaid chrominance component from said TV signal; a linear phase low passfilter coupled to said second means for applying said chrominancecomponent of the TV signal to an input terminal thereof, and having anoutput terminal, said low pass filter substantially rejecting thefrequency spectrum normally occupied by said chrominance component; andthird means for combining the luminance component of signal with the lowpassed filtered signal at the output terminal of said low pass filter tocorrect the luminance component of signal for errors therein due tointerimage motion.
 3. The comb filter set forth in claim 2 wherein thethird means comrprises a linear addition circuit.
 4. The comb filter setforth in claim 2 wherein the third means comprises a linear subtractioncircuit.
 5. A method of comb filtering TV signals to separatechrominance and luminance components thereof comprising:delaying the TVsignal by the duration of one frame of information; subtractivelycombining delayed and non-delayed TV signal to produce a comb filteredchrominance component; additively combining delayed and non-delayed TVsignal to produce a comb filtered luminance signal; low pass filteringthe combed chrominance component with a linear phase filter tosubstantially eliminate the chrominance signal therefrom; and combiningthe low pass filtered combed chrominance signal with the combedluminance signal during periods of interframe motion to produce a combedluminance signal substantially free of distortion due to interframescene motion.